The art of crystal growing as it relates to electronic semiconductors and the like has placed a special emphasis on the growth of large substantially perfect crystals. The size of the crystal determines the size of the electronic circuits that can be made from it. One case in point involves optical detector arrays. The larger the array the better can be the resolution of a given image projected thereon. Large single crystals also are valuable for tools, standards and for their aesthetic qualities, and the present application is directed to all of these areas.
One type of crystal that appears to be particularly promising, at least in the field of optical detectors is cadmium telluride (CDTe). The apparati and methods used to date for the growth of CdTe have not been successful in producing large sized, i.e. greater than about 2 cm diameter single crystals of high purity and stoichiometry. The growth habits of CdTe are best described as being inconsistent. A boule or continously grown body of CdTe will exhibit precipitates, lattice defects, multiple crystal growth, and twinning. Four prior art methods and their advantages with regard to CdTe crystals are listed below.
a. The Czochralski method; J. Czochralski, Z. Physik. Chem 92, 219 (1917), J. B. Mullin and B. W. Straughan, Revue de Physique Applique, 12, 105 (1977); is the growth of CdTe on a CdTe seed crystal which is partially immersed in the CdTe melt. The seed is rotated in order to facilitate solution stirring and is slowly drawn upwards from the melt as the crystal is grown. In order to eliminate or minimize the vapor losses of CdTe at temperatures of about 1100.degree. C., the crystal is grown in a high pressure chamber or the melt is encapsulated with B.sub.2 O.sub.3, a material that exhibits very low vapor pressure. CdTe that is grown by this method exhibits poor crystalline qualitites, i.e. excessive low angle grain boundaries, lamellar twins, dislocations and precipitates.
b. The vertical Bridgman technique; N. R. Kyle, J. Electrochem. Soc. 118, 1790 (1971); is the planar solidification of a material from the molten state. Stoichiometrically prepared CdTe is sealed in a two-inch diameter fused silica crucible and heated to approximately 1120.degree. C. The crucible is lowered at about 0.5 cm per hour through a solidification zone of about 1090.degree. C. A 10 cm (in length) boule of CdTe can be grown in approximately 24 hours. The boule consists of large domains of single crystals, twin planes, inclusions, and microcrystalline material. A boule will normally yield 1.times.1.times.2 cm crystals.
c. The traveling heater method; S. Brelant, M. Elliott, G. Entine and S. Hsu, Rev. Phys. Appl. 12, 141 (1977); is a narrow zone dissolution-regrowth technique. A one-cm diameter CdTe rod is sealed in a fused silica ampoule containing a prescribed amount of Te at the bottom. The tube is slowly lowered through a 0.5-1.0 cm wide resistance zone heater which is at about 700.degree. C. As the rod slowly moves downward through the hot zone the upper surface of molten Te dissolves CdTe, and the lower surface, being cooled by ambient, the CdTe regrows in crystalline form. The rod is lowered at a rate of approximately 5 mm per day. The travel rate is limited primarily by the diffusion time of the dissolved CdTe in the molten zone for recrystallization. The boundaries and crystal size is limited in diameter, i.e. 1 cm and is not free of grain boundaries and precipitates.
d. The solution growth method; K. Zano, J. Electron. Matl. 3, 327 (1974); is the crystallization and growth of CdTe in a Te-rich solution. The material is sealed in a 2-inch flat-bottomed quartz crucible and heated to 900.degree. C. A "cold finger" is applied to the center of the bottom of the crucible and thus becomes the site for nucleation. The thermal gradients induced by the cold finger enables the liquid to circulate over the growth site. As the CdTe grows the solution becomes more Te-rich and thus is cooled at a rate of about 4.degree. C. per hour to a temperature of 650.degree. C. The crystal is grown in a "cake" form at the bottom with the (110) plane perpendicular to the base of the crucible. The cake still exhibits the undesirable properties described in methods a, b, and c, but due to the circulation of the solution and lower growth temperatures, the crystals exhibit smaller volumes of precipitates and lesser numbers of grain boundaries and twin planes than the other methods. A crystal can be grown in about two days.